Optical disk unit

ABSTRACT

An optical disk unit is incidence a light spot upon an optical disk. The light spot is radially moved to cross tracks formed on the optical disk. The tracks are provided with specific access marks. The number of access marks passed by the light spot as well as the number of tracks crossed by the light spot until a change is found in a detected access mark are counted. The latter number is divided by the former number and multiplied by a coefficient to provide a speed signal indicative of a speed of the light spot relative to the optical disk.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical disk unit that records andreads information to and from an optical disk, and particularly to anoptical disk unit that can accurately measure the relative speed of alight spot radially moving on the optical disk to access a target trackon the optical disk.

2. Description of the Prior Art

An optical disk unit employs a light spot to access one of a pluralityof tracks formed on an optical disk. This track accessing operation isgenerally carried out by a combination of speed control and positioncontrol.

In speed control, a relative speed of the light spot radially moving onthe optical disk by crossing the tracks is measured, and compared with areference speed to provide a speed error signal. Based on the speederror signal, the speed of the light spot is controlled.

When the light spot reaches a target track, the position control isstarted to make the light spot follow the target track. In positioncontrol, a positional error signal is provided based on an error betweenthe position of the light spot and target track. According to thepositional error signal, the position of the light spot is controlled.

FIG. 1 shows a movement of the light spot on the optical disk. To accessa target track where required data is recorded, the light spot is movedfrom a present track to the target track according to the speed controlat first. When the light spot reaches the target track, the light spotis slowed down, and the speed control is switched to the positioncontrol.

To measure a relative speed of the light spot on the optical disk forthe speed control, a conventional technique detects a signal from theoptical disk, and generates a pulse whenever the light spot crosses oneor a half track. An interval of pulses thus obtained corresponds to therelative speed of the light spot. The conventional technique measures atime period of the interval, and executes arithmetic operationsincluding a multiplicative inverting operation to find the relativespeed of the light spot.

FIG. 4 shows a principle of the conventional speed measuring technique.In the figure, a curved line indicates temporal changes of an actualrelative speed of the light spot, and a straight line a measuredrelative speed of the same.

Pulses whose intervals change in response to the relative speed of thelight spot are generated at t1, t2, and so on, as the light spot ismoved on the optical disk. When a pulse is generated at t1, a timemeasurement starts. When the next pulse is generated at t2, an elapsedtime (t2-t1) for the period from t1 to t2 is provided and held. Aninverse number of the elapsed time for the period of (t2-t1) iscalculated and multiplied by a proper coefficient to provide a speed d2,which is an averaged speed of the light spot moved relative to theoptical disk for the period of (t2-t1). The speed d2 is held for aperiod from t2 to t3. This operation is repeated for each pulsegeneration to determine the relative speed of the light spot on theoptical disk.

The technique mentioned above is relatively easy to provide the relativespeed of a light spot. The technique has, however, several drawbacks. Asshown in FIG. 4, the measured speed indicated satisfactorily follows theactual speed only before t4. After t4, the measured speed greatlydeviates from the actual speed. The reason for this is because themeasured speed is not updated frequently in a deceleration state wherean interval of pulses gradually gets longer. In an extreme case ofbeyond t7, a speed d7 measured at t7 is maintained forever, if no pulseis generated after t7, even when the actual speed becomes 0 after t7. Ifsuch an incorrect measurement is used for controlling the light spot, aphase delay may occur which destabilizes a motion of the light spot.

One technique for providing information such as the relative speed of alight spot for various control purposes is a sample servo technique,which will be explained with reference to FIGS. 2 and 3.

An optical disk 1 shown in FIG. 2 has spiral or concentrical tracks 2.Servo areas 3 are intermittently formed on the tracks 2 to providedetection signals from which information for various control purposes isobtainable. FIG. 3 shows an example of the servo area 3. The servo area3 of each track 2 involves wobbled pits 7 and 8 disposed on the left andright sides of the track, an access mark portion 6, a mirror portion 4,and a clock pit 5. The wobbled pits 7 and 8 provide tracking signals,and the mirror portion 4 provides a focus error signal. The clock pit 5provides a system clock to be used for recording and reading operations.

The access mark portion 6 provides a pit pattern that is specific forpredetermined tracks and is utilized to detect the number of trackscrossed by a light spot radially moving on the optical disk 1, in atarget track accessing operation. FIG. 3 shows 16 pit patterns thatrepeat every 16 tracks.

FIG. 5 shows a conventional optical disk unit for achieving the sampleservo technique explained above. In the figure, a laser beam source 10emits a laser beam, which passes through a collimator lens 11, beamsplitter 12, and objective lens 13, and the beam spot is incidence uponan optical disk 14. A reflection of the light spot from the optical disk14 is passed through the objective lens 13, reflected by the beamsplitter 12, and led to a photodetector 15.

The photodetector 15 provides an output signal 16, which is amplified bya preamplifier 17, and led to a read signal processing circuit (notshown) and a waveform shaping circuit 18, which provides a binary signal19. The signal 19 is transferred to a clock generator 20, whichregenerates a system clock 21 based on a clock pit of a servo area ofthe optical disk 14.

A timing signal generator 22 uses the system clock 21 to provide atiming signal 23 to be used for detecting an access mark from the binarysignal 19. A crossed tracks number detector 24 uses the timing signal 23to detect the access mark from the binary signal 19, and provides acrossed tracks number 25 when a change is detected in the access mark.The crossed tracks number indicates the number of tracks crossed by thelight spot. The crossed tracks number 25 is supplied to a storagecircuit 27 through a latch circuit 26, and multiplied by a coefficientto provide a speed data signal 28. The signal 28 is converted by adigital-to-analog converter 29 into a speed signal 30.

FIG. 6 shows relative speed characteristics measured by the conventionaloptical disk unit explained above. In the figure, an abscissa representsactual speeds, and an ordinate measured speeds. Values shown in thefigure are based on a period of appearance of the servo areas, i.e., asample period, of 20 μs and a track pitch of 1.6 μm. An actual speed inthe range of 0 to 0.08 m/s will be measured as 0 or 0.08 m/s, and anactual speed in the range of 0.08 to 0.16 m/s will be measured as 0.08or 0.16 m/s. The reason of this is because resolution for the accessmarks is one track to cause a maximum error of 0.08 m/s.

A reading operation of the access marks in the servo areas by theconventional optical disk unit will be explained with reference to FIGS.7a, 7b, and 7c.

FIG. 7a shows access marks of tracks A and B, FIG. 7b output signalsbased on the access marks of FIG. 7a, and FIG. 7c binary signals shapedfrom the output signals of FIG. 7b. The optical disk unit is storingsixteen kinds of reference binary patterns with which a detected patternis checked to see whether it matches one of the reference binarypatterns.

Supposing the binary pattern Qa of FIG. 7c is received, it is recognizedthat a light spot is on the track having the access mark A because thereceived signal matches one of the stored patterns. By comparing accessmarks read at two consecutive sample times with each other, the numberof tracks crossed by the light spot during the sample period will bedetected. For example, it is detected that the crossed tracks number isone from the access marks A and B. In way, if a pattern that matches oneof the 16 reference access mark patterns is detected, the number oftracks crossed by the light spot can be detected.

The light spot on the optical disk has a certain size, and therefore,signals such as a and b shown in FIG. 7b are sometimes detected betweenthe tracks A and B, although the detected level thereof is low.

If such signals are obtained between the tracks, the signals may providea combined pattern a-b or Qa-b shown in FIGS. 7b and 7c. Since thecombined pattern does not match with any one of the stored patterns, itis judged that a read error has occurred, and no operation is carriedout to find the crossed tracks number. Namely, a correct crossed tracknumber will not be obtainable until a matching pattern is detected. Thismay reduce the chance of obtaining information for controlling themovement of the light spot, and lead to an insufficient control of theaccessing operation.

The conventional optical disk unit has another problem of providingpattern signals alternately. When output signals from an optical diskare unstable or when tracks of the optical disk are eccentric relativeto the center of the optical disk, binary pattern signals such as Qa andQb of FIG. 7c may alternately be detected even when the light spot isstationary between the tracks A and B. This may happen during anaccessing operation in which the light spot crosses a track in severalsample periods, and particularly in a latter half of the accessingoperation. If this happens, it is judged that the light spot is movingirregularly back and forth on the optical disk, and then, the light spotwhich is actually moving smoothly in one direction will wrongly becontrolled.

As explained above, the light spot on the optical disk has a certainsize, and therefore, detects signals between adjacent tracks of theoptical disk. This may cause the following problems:

(1) A combined pattern such as Qa-b of FIG. 7c, if detected, is judgedas a read error, and the optical disk unit carries out no operation offinding a crossed tracks number. This reduces a chance of correctcontrol of the light spot, and destabilizes the movement of the lightspot on the optical disk.

(2) Alternate detection of binary patterns from adjacent tracks alsodestabilizes the movement of the light spot on the optical disk, becausethe optical disk unit will judge from the alternate detection that thelight spot is moving alternately in forward and backward directions, andunnecessarily control the movement of the light spot according to thejudgment, even when the light spot is moving smoothly in one directionon the optical disk.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an optical disk unitthat can achieve a stable speed control of a light spot in accessing atrack on an optical disk.

Another object of the present invention is to provide an optical diskunit that can accurately detect the speed of a light spot relative to anoptical disk.

Still another object of the invention is to provide an optical disk unitthat can correctly measure the speed of a light spot relative to anoptical disk even in a decelerating state of the light spot.

Still another object of the invention is to provide an optical disk unitthat can provide improved controllability for a high-speed accessingoperation with minute reading error of servo information.

According to an aspect of the present invention, an optical disk unit isincidence upon an optical disk with a light spot. The light spot ismoved to cross tracks formed on the optical disk. The tracks areprovided with specific access marks. The number of access marks passedby the light spot as well as the number of tracks crossed by the lightspot until a change is found in a detected access mark is counted. Thelatter number is divided by the former number and multiplied by acoefficient to provide a signal indicative of a speed of the light spotrelative to the optical disk.

According to another aspect of the present invention, an optical diskunit comprises a pulse train generator for generating a train of pulseswhose intervals change in response to changes in a speed of a light spotthat is moved relative to an optical disk in a radial direction of theoptical disk; a counter for counting an elapsed time of a pulsegenerated by the pulse train generator; a memory for storing an outputof the counter until the pulse train generator generates the next pulse;and an arithmetic circuit for comparing an output of the counter with anoutput of the memory, and selecting larger one of the outputs. A speedof the light spot relative to the optical disk is measured according tothe selected output.

According to still another aspect of the present invention, an opticaldisk unit employs conventionally used on-track access mark patterns and,in addition, intra-track access mark patterns. The on-track access markpatterns are based on access marks formed on tracks of an optical disk,and the intra-track access mark patterns are based on patterns that maybe detected between adjacent tracks as combinations of the on-trackaccess marks of the adjacent tracks. When a detected access mark patternmatches one of the intra-track access mark patterns, the optical diskunit of the present invention recognizes that the light spot is locatedbetween the adjacent tracks. Namely, the unit of the present inventiondecreases a read error due to the detection of the intra-track accessmark pattern.

These and other objects, features and advantages of the presentinvention will be more apparent from the following detailed descriptionof preferred embodiments in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view for explaining speed control of a light spot in a dataaccessing operation;

FIG. 2 is a schematic view showing a sample servo format optical disk;

FIG. 3 is a view showing servo patterns formed on the optical disk ofFIG. 2;

FIG. 4 is a view showing light spot moving speeds measured by aconventional technique;

FIG. 5 is a block diagram showing an optical disk unit according to aprior art;

FIG. 6 is a view showing characteristics of light spot moving speedsmeasured by the prior art of FIG. 5;

FIGS. 7a to 7c are views showing pit arrangements of an access markportion of an optical disk, and binary signals derived from the pitarrangements;

FIG. 8 is a block diagram showing an optical disk unit according to afirst embodiment of the present invention;

FIGS. 9a to 9c are views showing actual speed characteristics of a lightspot, speed characteristics of the light spot measured by the prior art,and speed characteristics of the light spot measured by the presentinvention, respectively;

FIG. 10 is a view showing characteristics of light spot moving speedsmeasured by the first embodiment;

FIG. 11 is a block diagram showing an optical disk unit according to asecond embodiment of the present invention;

FIGS. 12a to 12e are views showing light spot moving speeds measured bythe second embodiment;

FIG. 13 is a view showing light spot moving speeds measured by amodification of the second embodiment;

FIGS. 14a and 14b are views showing access marks stored in a binarypattern memory of an optical disk unit according to a third embodimentof the present invention, and waveforms of output signals derived fromthe access marks; and

FIG. 15 is a block diagram showing a crossed tracks number detectingcircuit of the third embodiment.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 8 is a block diagram showing an optical disk unit according to thefirst embodiment of the present invention.

The optical head 9 comprises a laser beam source 10, a collimator lens11, a beam splitter 12, an object lens 13, and a photodetector 15. Thelaser beam source 10 is, for example, a semiconductor laser, and emits alaser beam. The laser beam is passed through the collimator lens 11 andbeam splitter 12, and converged by the object lens 13 into a very smalllight spot, which is incident upon the optical disk 1.

Reflection of the light spot from the optical disk 1 is passed throughthe object lens 13, reflected by the beam splitter 12, angled to thephotodetector 15, which provides an output signal 16. The output signal16 is amplified by a preamplifier 17 and supplied to a read signalprocessing circuit (not shown) and a waveform shaping circuit 18, whichprovides a binary signal 19 to a clock generator 20. The clock generator20 provides a system clock 21 based on a clock pit included in a servoarea of the optical disk 1. Based on the system clock 21, a timingsignal generator 22 generates a timing signal 23 to be used fordetecting an access mark pattern from the binary signal 19.

A crossed tracks number detector 24 uses the timing signal 23 to detectthe access mark pattern from the binary signal 19, and provides acrossed tracks number 25 according to a change detected in the accessmark pattern. If no change is detected in the access mark pattern, thedetector 24 provides a crossed tracks number of 0. The detector 24 alsoprovides an access mark passing pulse 31 whenever the light spot passesover an access mark. Even when the access mark is not correctly read,the access mark passing pulse 31 is provided every time when the lightspot passed over one access mark on the optical disk 1.

The crossed tracks number 25 is transferred to a latch circuit 26. Thelatch circuit 26 holds the crossed tracks number 25 until the nextcrossed tracks number is provided thereto, and provides a crossed tracksnumber 25A corresponding to the crossed tracks number 25 to a zerodetector 32 and an arithmetic circuit 38.

The zero detector 32 judges whether or not the crossed tracks number 25Ais zero. If it is not zero, the zero detector 32 provides a trackcrossed signal 33 to a delay circuit 34 and a gate circuit 42.

The delay circuit 34 properly delays the track crossed signal 33 to, forexample, about one tenth of a sample period of the access mark passingpulse 31, and provides a reset signal 35 to reset a counter 36.

The counter 36 counts the number of access mark passing pulses 31provided by the crossed tracks number detector 24, and provides a samplecount 37 to the arithmetic circuit 38. The sample count 37 representsthe number of access marks of the same track passed by the light spot.Namely, the sample count 37 corresponds to a time period during whichthe light spot was on the same track.

The arithmetic circuit 38 divides the crossed tracks number by thenumber of access marks passed by the light spot, and provides a speeddata signal 39 that is proportional to a result of the division. Thearithmetic circuit 38 comprises, for example, a ROM which is accessed byan upper address designated by the crossed tracks number 25A and a loweraddress designated by the sample count 37 to provide the speed datasignal 39 which represents a value derived by dividing the crossedtracks number by the sample count and multiplying a result of thedivision by the coefficient.

The speed data signal 39 is supplied to a digital-to-analog converter 40when the gate circuit 42 allows the same upon receiving the trackcrossed signal 33. The converter 40 converts the speed data signal intoan analog speed signal 41.

Operation of the above arrangement will be explained.

The crossed tracks number detector 24 detects an access mark pattern andprovides a crossed tracks number 25 representing the number of trackscrossed by the light spot on the optical disk 1. The crossed tracksnumber detector 24 also provides an access mark passing pulse 31whenever the light spot passes over one access mark on the opticaldisk 1. According to the crossed tracks number 25, the latch circuit 26provides a crossed tracks number 25A. The crossed tracks number 25A istransferred to the zero detector 32 and arithmetic circuit 38.

The zero detector 32 judges from the crossed tracks number 25A whetheror not the number of tracks crossed by the light spot is zero. If thenumber is not zero, the zero detector 32 provides a track crossed signal33 to the delay circuit 34 and gate circuit 42. The delay circuit 34properly delays the track crossed signal 33, and provides a reset signal35 to the counter 36.

Meantime, the counter 36 counts the number of access mark passing pulses31 supplied by the crossed tracks number detector 24, and provides asample count 37 to the arithmetic circuit 38. A count of the counter 36is reset by the reset signal 35 from the delay circuit 34. Namely, thecounter 36 is reset whenever a change is found in a detected accessmark. This means that a count of the counter 36 will be large when arelative speed of the light spot is slow, and small when the same isfast. When the light spot relative speed is very fast, every detectedaccess mark may be different from the previously detected access mark,and therefore, the counter 36 may count only 1.

Based on the crossed tracks number 25A and sample count 37, thearithmetic circuit 38 divides the number of tracks crossed by the lightspot by the number of access marks passed by the light spot, andprovides a speed data signal 39 proportional to a result of thedivision. The speed data signal 39 is supplied to the digital-to-analogconverter 40 under the control of the gate circuit 42 when a change isfound in a detected access mark, and converted into an analog speedsignal 41.

FIG. 10 shows speed measurement characteristics of the embodiment, inwhich an abscissa indicates actual speeds and an ordinate detectedspeeds, with a sample period of 20 μs and a track pitch of 1.6 μm. Whenan actual relative speed of the light spot is 0.05 m/s, the embodimentmay detect a speed of 0.04 m/s or 0.08 m/s. Namely, an error of theembodiment will be 0.03 m/s at maximum. Compared to this, theconventional technique detects a speed of 0 or 0.08 m/s for the sameactual speed, to provide an error of 0.05 m/s at maximum as shown inFIG. 6.

As is apparent from FIG. 10, the embodiment can greatly reduce an errorin detecting the relative speed of a light spot on an optical disk,particularly in a slow speed state of the light spot speed. Accordingly,the embodiment can improve, in a data accessing operation, a speeddetecting resolution of the light spot particularly in an important slowspeed state, thereby improving a speed controlling accuracy of the lightspot and realizing a high-speed access.

As explained above, the first embodiment of the present invention countsthe number of access marks passed by a light spot as well as the numberof tracks crossed by the light spot until a change is found in adetected access mark, divides the latter number by the former number,and provides a speed signal proportional to a result of the division.With a simple structure, the first embodiment can remarkably reduce anerror in the measurement of the light spot relative speed, particularlyin a slow speed state, compared to the conventional technique. The firstembodiment thus improves a controllability and track tracing performanceof the light spot, and reduces an access time.

The present invention can effectively control the light spot movementeven when the light spot crosses several tracks in one sample period.This will be explained next.

When the light spot crosses several tracks in one sample period, anangle between a track longitudinal direction and a light spot movingdirection becomes larger. Namely, a crossing angle of the light spotwith respect to an access mark becomes larger, so that the access markmay be read obliquely to cause a read error. If the read error occurs, acrossed tracks number at the sample timing is not provided correctly.This may deteriorate an accuracy of speed control of the light spot.

If such a read error occurs, the crossed tracks number detector 24 ofthe embodiment of FIG. 8 provides a crossed tracks number of 0. When anaccess mark is correctly read next time, the crossed tracks numberdetector 24 provides a crossed tracks number including tracks crossedduring the read error period. During this read error period, the crossedtracks number detector 24 continuously provides an access mark passingpulse 31 whenever the light spot passes over one access mark, so thatthe arithmetic circuit 38 may correctly calculate an average speed ofthe light spot during the period.

FIG. 9a shows a light spot actual speed detected with no read error.FIG. 9b shows the same light spot speed but detected by the conventionaloptical disk unit. It is apparent that FIG. 9b greatly differs from FIG.9a. FIG. 9c shows the same light spot speed detected according to thepresent invention. FIG. 9c is averaged at a speed changing portion andanalogous to the correct detected speed of FIG. 9a.

The above features of the present invention are realized by the crossedtracks number detector 24 for finding a change in a detected access markand providing the number of tracks crossed by the light spot, thecounter 36 for counting the number of access marks passed by the lightspot until the change in the detected access mark is found, and thearithmetic circuit 38 for providing a speed signal corresponding to avalue obtainable by dividing the crossed tracks number by the count ofthe counter 36.

An optical disk unit according to the second embodiment of the inventionwill be explained with reference to FIGS. 11 to 13.

The optical head 9 includes a laser beam source 10, a collimator lens11, beam splitter 12, an object lens 13, and a photodetector 15. Thelaser beam source 10 such as a semiconductor laser emits a laser beam.The laser beam is passed through the collimator lens 11 and beamsplitter 12, and converged by the object lens 13 into a very small lightspot, which is incidence upon the optical disk 1.

A reflected light beam from the optical disk 1 is passed through theobject lens 13, reflected by the beam splitter 12, and led to thephotodetector 15, which provides a detected signal. The detected signalis amplified by a preamplifier 17 and supplied to a read signalprocessor (not shown) and a waveform shaping circuit 18, which providesa binary signal to a clock generator 20.

Based on the binary signal, the clock generator 20 generates a systemclock that corresponds to a clock pit 5 (FIG. 3) included in a servoarea formed on the optical disk 1. The system clock is supplied as asystem clock to the read signal processor (not shown) and a timingsignal generator 22, which generates various timing signals according tothe system clock, required by various components.

The amplified signal from the preamplifier 17 is also supplied to asample and hold circuit 50. In response to a sample timing signal S1from the timing signal generator 22, the sample and hold circuit 50samples and holds a peak value of a signal corresponding to the clockpit in the amplified signal. An output of the sample and hold circuit 50is supplied to a waveform shaping circuit 51, which provides a binarysignal one period of which corresponds to one track.

An edge detector 52 may be a monostable multivibrator, and generates athin pulse at every edge of a rise or fall of the binary signal providedby the waveform shaping circuit 50, thereby providing a train of pulseswhose intervals change in response to a relative speed of the light spotradially moving on the optical disk 1.

The pulse train from the edge detector 52 is applied to a reset input Rof a time counter 53. A clock input CK of the time counter 53 receives atiming signal from the timing signal generator 22. An interval of thetiming signals corresponds to a sample period. When the reset input R ofthe time counter 53 receives a pulse, the time counter 53 is reset.Thereafter, the time counter 53 counts up the number of pulses appliedto the clock input CK thereof. Namely, whenever the edge detector 52provides a pulse, the time counter 53 starts to count a sample number,and provides a digital output indicative of the sample number. Bymultiplying the sample number by the sample period, an elapsed time willbe obtained. Therefore, an output of the time counter 53 just beforereset indicates an interval of pulses provided by the edge detector 52.

An output pulse of the edge detector 52 is provided as a latch timingsignal to a latch 54 acting as storage means. Upon receiving the latchtiming signal, the latch 54 stores a digital output of the time counter53. Namely, the latch 54 holds a count value of the time counter 53 justbefore the time counter 53 is reset.

Outputs of the time counter 53 and latch 54 are provided to a comparator55 and a selector 56. The comparator 55 compares the outputs of the timecounter 53 and latch 54 with each other, and provides a binary signalindicating which of them is larger than the other.

A selector 56 receives the binary output of the comparator 55 as aselect signal, selects larger one of the outputs of the time counter 53and latch 54, and provides the selected one to a speed calculator 57.

The speed calculator 57 calculates an inverse number of the output valueof the selector 56, and multiplies the inverse number by a coefficientthat is set according to the sample period, track pitch, etc., therebyproviding a relative speed of the light spot moving radially on theoptical disk 1. The speed calculator 57 then provides a digital outputcorresponding to the relative speed of the light spot to adigital-to-analog converter 58.

The digital-to-analog converter 58 converts the digital output into aspeed signal V1, which is provided to a differential amplifier 63, inwhich the signal V1 is subtracted from a reference speed signal V2.

The reference speed signal V2 indicates a reference value of therelative speed of the light spot in a track accessing operation. Thereference speed signal V2 is based on a detection signal of an accessmark portion 6 of a servo area 3 of the optical disk 1, and generated byan access mark pattern extracting circuit 59, memory 60, track counter61, and a reference speed detector 62.

Namely, an output of the waveform shaping circuit 18 is provided to theaccess mark pattern extracting circuit 59, which extracts a binarypattern, i.e., an access mark pattern, according to the detection signalof the access mark portion 6 at every sample timing. The extractedaccess mark pattern is provided to the memory 60 and track counter 61.The track counter 61 compares the access mark pattern just extractedwith an access mark pattern extracted at preceding sample timing andstored in the memory 60, finds the number of tracks crossed by the lightspot during an interval between the two sample timings, accumulates thecrossed tracks number, and provides the total number of tracks crossedtill now by the light spot in the track accessing operation.

The reference speed detector 62 subtracts the total track numberobtained by the track counter 61 from a track number set at the start ofthe track accessing operation as the number of tracks which must becrossed by the light spot to reach a target track, thereby finding thenumber of remaining tracks up to the target track. The reference speeddetector 62 then generates the reference speed signal V2 correspondingto the number of remaining tracks. The reference speed detector 62 has aROM table that is storing relations of the numbers of remaining tracksand optimum reference speeds, and finds an optimum reference speed inthe ROM table for a given remaining track number to provide the analogreference speed signal V2.

The differential amplifier 63 amplifies the difference (V2-V1) betweenthe measured speed signal V1 and the reference speed signal V2, andprovides a speed error signal EV indicative of an error of the measuredspeed. The speed error signal EV is supplied to a servo circuit 67.

The servo circuit 67 also receives a positional error signal EP that isindicative of a positional error of the light spot relative to thetarget track in the radial direction of the optical disk 1. Thepositional error signal EP is generated by sample and hold circuits 64and 65, and differential amplifier 66. The sample and hold circuits 64and 65 receive sample timing signals S2 and S3, respectively, from thetiming signal generator 22, to sample and hold signals corresponding tofirst and second wobbled pits 7 and 8 from the detection signal providedby the preamplifier 17. The differential amplifier 66 amplifies thedifference of outputs of the sample and hold circuits 64 and 65, andprovides the positional error signal EP.

When a distance from a start track to the target track is relativelylong, the track accessing process can generally be divided into anacceleration period during which the relative speed of the light spot isgradually increased, a constant speed period during which the relativespeed of the light spot is kept constant until the light spot is broughtclose to the target track, and a deceleration period during which therelative speed of the light spot is decelerated in the vicinity of thetarget track. During these periods, the relative speed of the light spotis controlled according to the speed error signal EV. When the lightspot reaches the target track, the light spot is controlled according tothe positional error signal EP to trace the target track.

To achieve such track accessing control of the light spot, the servocircuit 67 selects one of the speed error signal EV and positional errorsignal EP according to a mode switching signal MS, and provides theselected one through a phase compensating circuit, etc., to an actuatordriver 68. The actuator driver 68 amplifies the output signal of theservo circuit 67 to provide an appropriate signal to an actuator 70.

With reference to time charts of FIGS. 12a to 12e, an operation of theembodiment of the present invention for measuring the relative speed ofthe light spot that is moving radially on the optical disk 1 will beexplained in detail.

In FIG. 12a, a curved line indicates temporal changes of actual relativespeed of the light spot, a dotted line the same speed but measured bythe conventional optical disk unit, and a solid uncurved line the samespeed but measured by the embodiment of the present invention. FIG. 12bshows a sample timing signal S1, FIG. 12c a sample and hold signal ofthe clock pit provided by the sample and hold circuit 50 according tothe sample timing signal S1, FIG. 12d a binary signal provided by thewaveform shaping circuit 51, and FIG. 12e waveform of a pulse trainprovided by the edge detector 52.

As shown in the figures, pulses provided by the edge detector 52 aresynchronous to the sample timing signal S1. Each pulse of the edgedetector 52 is given to the reset input R of the time counter 53. Theclock input CK of the time counter 53 receives the sample timing signalS1. Accordingly, whenever the edge detector 52 provides a pulse, thetime counter 53 measures an elapsed time after the generation of thepulse until the next sample timing, as a multiple of a cycle τ of thesample timing signal S1.

During the acceleration and constant speed periods, an output of thelatch 54 is larger than or equal to an output of the time counter 53.The selector 56 selects, therefore, the output of the latch 54 accordingto an output of the comparator 55. Namely, the selector 56 selects, atthe present sample timing, a time difference between the present andpreceding pulses generated by the edge detector 52, and provides thesame to the speed calculator 57.

In the decelerating period, the time counter 53 measures an elapsed timefrom a pulse generated by the edge detector 52 at the present sampletiming until the next sample timing, as a multiple of the cycle τ of thesample timing signal S1, thereby estimating the relative speed of thelight spot for the preceding sample timing. This operation will beexplained for a time period between t4 and t5 of FIG. 12. Each intervalbetween t4, t41, t42, and t43 is one sample period τ.

(1) At t4, edge detector 52 generates a pulse, and the time counter 53counts "1" at t4. Meantime, according to the pulse generated by the edgedetector 52, the latch 54 latches at t4 the number "3" of the sampletiming signals S1 for the time period between t3 and t4. At this moment,the comparator 55 judges that the output of the latch 54 is larger thanthe output of the time counter 53, so that the selector 56 selects theoutput "3" of the latch 54, and provides the selected one to the speedcalculator 57.

(2) At the next sample timing t41, the edge detector 52 does notgenerate a pulse, so that the latch 54 keeps the number "3" as it is,and the time counter 53 increases its count to "2" at t41. In this casealso, the comparator 55 judges that the output of the latch 54 is largerthan the output of the time counter 53, so that the selector 56continues to provide the output "3" of the latch 54 to the speedcalculator 57.

(3) At the next sample timing t42, the edge detector 52 does notgenerate a pulse, so that the latch 54 keeps the number "3" as it is,and the time counter 53 increases its count to "3" at t42. Since theoutputs of the time counter 53 and latch 54 are each "3", the selector56 continues to provide the value "3" to the speed calculator 57.

(4) At the next sample timing t43, the edge detector 52 does notgenerate a pulse, so that the latch 54 keeps the number "3" as it is,and the time counter 53 increases its count to "4" at t43. Then, thecomparator 55 judges that the output "4" of the time counter 53 islarger than the output "3" of the latch 54, and therefore the selector56 selects the output "4" of the time counter 53 and provides the sameto the speed calculator 57. As a result, the speed calculator 57provides a speed signal having a value smaller than a preceding value.The value of the relative speed signal is equal to a value for a timewhen the next pulse is provided by the edge detector 52.

As indicated with the solid uncurved line in FIG. 12a, the speedprovided by the speed calculator 57 can predict a reduction in theactual speed in the deceleration period. Namely, during the decelerationperiod, the present invention can satisfactorily measure the light spotrelative speed with small phase delay.

In the above explanation, the clock input CK of the time counter 53receives, as a time counting signal, the sample timing signal S1 fromthe timing signal generator 22. Instead, a faster system clock may beprovided to the clock input CK as a modification of the secondembodiment of the invention. In this case, a value counted by the timecounter 53 substantially continuously increase, so that relations of theactual speed and measured speed will be as shown in FIG. 13. Operationsof this modification in the acceleration and constant speed periods aresimilar to those of the second embodiment. According to an output of thecomparator 55, the selector 56 selects an output of the latch 54, i.e.,a time period counted between a last pulse and the preceding pulse thathave been generated by the edge detector 52, and provides the selectedone to the speed calculator 57.

An operations of the modification for the deceleration period from t4 tot5 will be explained.

When the edge detector 52 generates a pulse at t4, the latch 54 isholding a time count (t4-t3) for a period between t4 and t3, and thetime counter 53 starts to count an elapsed time from t4. Up tot4+(t4-t3), an output of the latch 54 is larger than that of the timecounter 53. After that, however, an output of the counter will be largerthan an output of the latch 54. Accordingly, according to an output ofthe comparator 55, the selector 56 selects the output of the timecounter 53, and provides the selected one to the speed calculator 57.Then, a measured speed calculated by the speed calculator 57 decreasesuntil the edge detector 52 generates a pulse, as indicated with thethick line of FIG. 13. This decreasing curve corresponds to changes inoutput values of the time counter 53 that gradually decrease in responseto the high-speed system clocks. In this way, the modificationsatisfactorily predicts the reduction in the light spot relative speedduring the deceleration period, and demonstrates only a little phasedelay.

The above explanations relate to the sample servo type optical diskunit. For a continuous servo type optical disk unit, track passingpulses obtainable from an optical disk may be used to predict areduction in the relative speed of a light spot during the decelerationperiod with a little phase delay.

According to the above embodiments, the speed of a light spot relativeto an optical disk in the radial direction thereof can be measuredsatisfactorily even for the deceleration period. Based on such speedmeasurement, the relative speed of the light spot can be controlledstably with a little phase delay in a track accessing operation.

An optical disk unit according to the third embodiment of the inventionwill be explained with reference to FIGS. 14 to 16.

FIG. 14 is a view showing access marks employed in the third embodiment.In FIG. 14a, numeral 70 denotes an access mark portion arranged in eachof servo areas that are intermittently arranged on tracks of an opticaldisk. The tracks are concentrical or spiral similar to those of FIG. 3.In this embodiment, each access mark portion involves two pits 73a and73b that are arranged on a track centerline 72. Combinations of the twopits 73a and 73b express 16 kinds of access marks A to P.

FIG. 14b is a view showing shaped binary access mark patterns derivedfrom the access marks of FIG. 14a. For example, the access mark Acorresponds to a binary access mark pattern Qa. A binary access markpattern Qa-b represents a combination of patterns Qa and Qb. This sortof combined pattern is usually obtained between adjacent tracks due toan influence of access marks of the adjacent tracks. In this thirdembodiment, the access marks A to P are so selected that combinations ofthe patterns differ from one another. Accordingly, patterns to be usedfor detecting the moving state of a light sot on an optical disk are 32in total, i.e., 16 on-track patterns and 16 intra-track patterns. Basedon these patterns, a position of the light spot can be detected.

FIG. 15 is a block diagram showing a crossed tracks number detectoraccording to the third embodiment. The detector of FIG. 15 is a portionfor processing some of information pieces read out of the servo areas ofthe optical disk, for the purpose of a track accessing operation. Thisportion cooperates with conventional recording and reading portions.

A signal obtained from a clock pit of the servo area is provided to aclock detecting and timing pulse generating portion 81, which providestiming signals indicated with dotted lines to a binary waveform shapingportion 82, pattern number detector 83, a memory 84, and a crossedtracks number detector 85.

The binary waveform shaping portion 82 receives an analog signal fromthe access mark portion 70 of the servo area. The binary waveformshaping portion 82 generates a binary signal according to the receivedanalog signal, and provides the same to the pattern number detector 83.

The pattern number detector 83 receives the binary signal as well as asignal from a binary pattern memory 90 which is storing all the 32patterns in the order of FIG. 14b. The pattern number detector 83 checksto see whether the received binary signal matches one of the stored 32patterns, and, if matched, provides a pattern number of the matchedpattern.

The memory 84 receives a signal from the pattern number detector 83.Data stored in the memory 84 can be updated by a latch circuit, etc.

The crossed tracks number detector 85 receives signals from the patternnumber detector 83 and memory 84, and calculates how many tracks arecrossed by the light spot on the optical disk during the sample period.

The tracks of the optical disk generally have addresses, which are readby an optical pickup (the light spot) of the optical disk unit inrecording and reading states. A position of the optical pickup (thelight spot) is, therefore, easily found from the addresses in therecording and reading states. In an accessing state, however, theaddresses are not readable. This is the reason why a control system suchas one shown in FIG. 15 is needed to detect a movement of the lightspot, i.e., the optical pickup.

When the access operation is started, the optical pickup reads theaccess mark portion 70 of the servo area of the optical disk. A readsignal "A" (an access mark pattern) is shaped by the binary waveformshaping portion 82, and provided as a binary signal to the patternnumber detector 83. The pattern number detector 83 compares the binarysignal with the 32 patterns stored in the binary pattern memory 90 tosee whether the signal A matches one of the patterns. Similarly, signalsare sequentially read from the access mark portions of the optical disk,and judged in the same manner.

When a read signal matches one of the patterns stored in the binarypattern memory 90, a pattern number of the matched pattern is providedto the memory 84 and crossed tracks number detector 85. For example, thesignal A of FIG. 14a has a pattern number of 1, and a signal P has apattern number of 16. These pattern numbers can provide a relativeposition of the light spot on the optical disk.

Supposing that the memory 84 holds the pattern number 1 of the signal Aand that the signal B is provided to the pattern number detector 83, thepattern number 1 of the signal A held in the memory 84 is transferred tothe crossed tracks number detector 85, and the pattern number 2 of thesignal B is newly held in the memory 84 and provided to the crossedtracks number detector 85.

The crossed tracks number detector 85 compares the pattern numbers ofthe signals A and B with each other to detect the number of trackscrossed by the light spot. In the case of the signals A and B, thenumber of tracks crossed by the light spot will be one because two (thepattern number of signal B) minus one (the pattern number of signal A)equals to one.

As explained above, the binary pattern memory 90 is storing thereference patterns, so that, by comparing each received signal patternwith the reference patterns, the relative position and moved distance ofthe light spot (the optical pickup) can easily be found. By sequentiallyupdating a signal held in the memory 84, the number of crossed tracksmay continuously be detected. For example, if a signal C is detected atthe next sampling time, the memory 84 provides the pattern number 2 ofthe presently holding signal B to the crossed tracks number detector 85,and newly holds the pattern number 3 of the signal C. Meantime, thecrossed tracks number detector 85 finds one as the number of crossedtracks.

If the signal C is read after the signal A, the crossed tracks numberdetector 85 finds the number of crossed tracks of two. If the signal Ais read after the signal B, the crossed tracks number detector 85 findsthe number of crossed tracks of minus one, which means that the lightspot has moved in a reverse direction.

In addition to the conventionally used on-track patterns Qa to Qp, theembodiment employs the intra-track patterns Qa-b to Qp-a. Accordingly,even if a combined pattern is detected due to the influence of adjacenttracks, the combined pattern does not cause a read error but providesinformation that the light spot is positioned between adjacent tracks onthe optical disk.

For example, if a signal E-F having a pattern number of 5.5 is readafter the signal A, the crossed tracks number detector 85 detects thenumber of crossed tracks of 4.5. Then, it is recognized that the lightspot has crossed 4.5 tracks and is presently positioned between adjacenttracks. With this sort of decimal positioning, the relative speed of thelight spot can be accurately measured from the number of revolutions ofthe optical disk and a moving state of the light spot, so that theoptical pickup can be correctly controlled is accessing a target track.

The pattern number of the signal E-F may be "4" corresponding to thesignal e, or "5" corresponding to the signal F. Namely, instead ofemploying the decimal positioning, any decimal position may be relatedto an integer pattern number of the adjacent track.

With this arrangement, intra-track information that may cause a readerror on the conventional unit, may be effectively utilized to increasechances of obtaining information from the optical disk and realize quickand correct control.

During an accessing operation in which a light spot crosses a track inseveral sample periods, a probability of reading a combined binarysignal of, for example, Qa-b between adjacent tracks of the access marksA and B may increase. In this case, the conventional optical disk unitirregularly provides the binary signals Qa and Qb.

According to the invention, however, a probability of irregularlyoutputting the binary signals Qa and Qb is greatly reduced. Theinvention can pick up the combined signal Qa-b, and treats the same as asignal which is irrelevant of a light spot movement. The inventionobtains information related to the movement of the light spot only fromthe on-track signals, and never detects erroneously that the light spotis irregularly moving back and forth. This will be explained in detail.

The pattern number detector 83 judges whether a detected signal is anon-track signal or an intra-track signal, and, if the intra-track signalthat differs by ±0.5 tracks from the previously read on-track signal isfound, does not provide the intra-track signal to the memory 84 andcrossed tracks number detector 85. Accordingly, the memory 84 andcrossed tracks number detector 85 hold the previous on-track signal asit is. It is then recognized that the light spot is moving in the samedirection as detected by the previous on-track signal.

It is possible that the binary signals Qa and Qa-b, or Qa-b and Qb arealternately read. In this case, the binary signal Qa-b is ignored indetecting the movement of the light spot, and it is recognized that thelight spot is moving in the same direction as detected according to thepreceding signal. If the preceding signal is Qa-b, a lastly obtainedon-track signal determines the movement of the light spot. If apresently obtained on-track signal is the same as the preceding on-tracksignal, the crossed tracks number detector 85 calculates "0" from thesesignals to indicate that the light spot has crossed no track.

In this way, the embodiment stores the 16 on-track patterns Qa to Qp andthe 16 intra-track patterns Qa-b to Qp-a having high probability ofbeing encountered, and compares them with a detected pattern, therebycorrectly detecting the number of tracks crossed by the light spot.

The numbers of crossed tracks obtained sequentially are accumulated by atrack counter (not shown) to find the total number of crossed tracksduring an accessing operation. The resultant total is always comparedwith the number of tracks to be crossed by the light spot to reach atarget track that has been set at the start of the accessing operation,thereby finding the number of remaining tracks to be crossed to reachthe target track. The optical disk unit has a control system in which anideal speed control pattern is programed, and controls the relativespeed of the light spot (the optical pickup) in response to the numberof the remaining tracks up to the target track. According to the controlsystem, (1) a speed of the light spot measured from the sample periodand the number of crossed tracks is always compared with (2) a referencespeed based on the ideal speed control pattern, and the light spot (theoptical pickup) is controlled to reduce the difference between thespeeds (1) and (2).

Although the embodiment has employed the 16 kinds of on-track patterns,more or less kinds of patterns may be employed to achieve the similarcontrol.

The embodiment can correctly control the movement of the light spot onthe optical disk even when a combined binary signal is generated byadjacent tracks, so that the size of the light spot on the optical diskis not so critical. During an accessing operation, the light spot willbe positioned very close to the next track if the size of the light spotis relatively large, to increase a possibility of detecting the combinedbinary signal. Even so, unlike the conventional optical disk unit, theembodiment which employs the intra-track signals will not erroneouslycontrol the motion of the light spot. This is the reason why theaccuracy of the light spot on the optical disk is not so critical forthe present invention. Even if the optical pickup has some focusingerrors, the invention can stably control the optical pickup. The opticaldisk unit of the invention is easy to control during the accessingoperation, and resistive to external vibration.

In summary, an optical disk unit according to the invention uses anoptical disk having concentrical or spiral tracks on which many pits forsample servo control are formed. The pits form access marks foridentifying individual tracks on the optical disk. A light spot isincidence upon the surface of the optical disk to detect the accessmarks to judge a motion of the light spot on the optical disk. If acombination of the access marks of adjacent tracks is detected, theoptical disk unit judges that the light spot is positioned between theadjacent tracks, thereby improving the controllability of the lightspot.

Various modifications will become possible for those skilled in the artafter receiving the teachings of the present disclosure withoutdeparting from the scope thereof.

What is claimed is:
 1. An optical disk unit for transmitting a lightspot upon a surface of an optical disk, obtaining a detection signalfrom a reflection of the light spot, moving the light spot in accordancewith the detection signal to a target track formed on the optical disk,and recording and reading information to and from the target track, theoptical disk unit comprising:edge detector means for generating a trainof pulses whose interval changes in response to the detection signal;time measuring means for starting to measure a pulse interval wheneverthe edge detector means generates a pulse; storage means for holding anoutput of the time measuring means until the edge detector meansgenerates a next pulse; comparator means for comparing an output of thetime measuring means with an output of the storage means and forgenerating an output when the pulse interval exceeds a preceding pulseinterval; selector means, responsive to the output of the comparatormeans, for selecting the output of the storage means during a period oftime when the pulse interval is shorter than the preceding pulseinterval and for selecting the output of the time measuring means whenthe pulse interval exceeds the preceding pulse interval; and speedcalculator means for calculating a relative speed of the light spotusing an output of the selector means such that when the pulse intervalexcess the preceding pulse interval, and the relative speed starts todecrease in accordance with the output of the time measuring meansbefore a next pulse is generated from the edge detector means, therelative speed of the light spot is measured during deceleration withlittle phase delay, the relative speed calculated by the speedcalculator means being used for moving the light spot.
 2. An opticaldisk unit as claimed in claim 1, further comprising:sample hold meansfor sampling and holding the detection signal in response to a sampletiming signal, and for providing a sample and hold signal; the edgedetector means receiving sample and hold signals from the sample holdmeans, and generating, in synchronism with sample timing signals, atrain of pulses whose intervals change in response to changes in aradial speed of the light spot relative to the optical disk.
 3. Anoptical disk unit as claimed in claim 2, wherein the time measuringmeans starts to measure an elapsed time as a multiple of an interval ofthe sample timing signals whenever the edge detector means generates apulse, and provides an output indicative of the elapsed time.
 4. Anoptical disk unit as claimed in claim 2, wherein the time measuringmeans measures a time period between a sample timing when the edgedetector means generates a pulse and a next sample timing as a multipleof an interval of the sample timing signals, and provides an outputindicative of the time period.